Mehtod for cardiac magnetic resonance imaging

ABSTRACT

The invention relates to a method for cardiac magnetic resonance imaging. The method comprises the steps of monitoring the ECG of a patient and detecting the occurrence of R-waves (RI, R 2 , R 3 ) in the ECG, acquiring a plurality of phase encoded MR signals (a 1-3 , b 1-3 , c 1-3 , d 1-3 , e 1-3 ) per cardiac cycle by subjecting the patient to a sequence of RF field gradient pulses, and reconstructing an MR image from at least a part of the MR signals (a 1-3 , b 1-3 , C 1-3 , d 1-3 , e 1-3 ). For providing an improved cardiac MRI method which enables the acquisition and reconstruction of high quality MR images of the heart in cases in which the heart cycle of the examined patient changes over time or is irregular because of arrythlmia, the invention proposes that the ECG of the patient is monitored continuously during acquisition of MR signals, wherein MR signals (d 1  to c 1 , b 2  to a 2 , C 3  to b 3 ) used for image reconstruction are retrospectively selected depending on the time interval between successive R-waves (R 1 , R 2 , R 3 ).

The invention relates to a method for cardiac magnetic resonanceimaging, the method comprising the following steps:

A) monitoring the ECG of a patient and detecting the occurrence ofR-waves in the ECG;

B) acquiring a plurality of phase encoded MR signals per cardiac cycleby subjecting the patient to a sequence of RF pulses and magnetic fieldgradient pulses;

C) reconstructing an MR image from at least a part of the MR signalsacquired in step B).

Furthermore, the invention relates to a device for magnetic resonanceimaging for carrying out this method and a computer program for amagnetic resonance imaging device.

In magnetic resonance imaging (MRI) pulse sequences consisting of RF andmagnetic field gradient pulses are applied to an object (a patient) togenerate magnetic resonance signals which are scanned in order to obtaininformation therefrom and to reconstruct images of the object. Since itsinitial development the number of clinical relevant fields ofapplication of MRI has grown enormously. MRI can be applied to almostevery part of the body, and it can be used to obtain information about anumber of important functions of the human body. The pulse sequencewhich is applied during a MRI scan determines completely thecharacteristics the reconstructed images, such as location andorientation in the object, dimensions, resolution, signal-to-noiseratio, contrast, sensitivity for movements, etcetera. An operator of aMRI device has to choose the appropriate sequence and has to adjust andoptimize its parameters for the respective application.

In the past, cardiac magnetic resonance imaging methods have been oflimited clinical value for several reasons. This is because the heart asa moving object is particularly difficult to image, especially if animaging plane is set in space with the heart moving in and out of theimaging plane. Cardiac MR imaging is especially difficult since thebreathing of the examined patient causes a periodic motion of the heartand other surrounding internal structures of the body of the examinedpatient. The imaging situation is further complicated by the beatingmotion of the heart which is added to the breathing motion. Bothmotions, heart motion and breathing motion, are present during therelatively long period of acquisition of MR signals and causeundesirable artifacts in the resulting images. Image quality may bedegraded because of motion blurring for example.

It is known that the beating motion of the heart is fastest duringsystole and relatively motionless during diastole, in which the heart isfully expanded. Thus, MR images reconstruced from MR signals acquiredduring a diastole provide the clearest images of the heart. Thebreathing motion in turn can be eliminated by simply asking the examinedpatient to hold his or her breath during the acquisition of MR signalsor by acquiring the MR signals during quiet breathing periods.

According to known methods for cardiac MR imaging, such as described forexample in the U.S. 2002/0077538 A1, the ECG of the examined patient ismonitored in order to synchronize the acquisition of MR signals with theheart cycle. The ECG signal is a repetitive pattern reflecting theelectrical activity of the patients heart. Each cardiac cycle beginswith a so-called R-wave (highest amplitude peak) in the ECG signalduring the systole period and ends with the diastole period almostwithout any electrical activity. In the mentioned publication it issuggested to monitor the heart rate of the patient prior to the actualimage acquisition and to determine a time interval between successiveR-waves. This time period obtained from the heart rate monitoring isused to prospectively estimate future intervals between successiveR-waves. The acquisition of MR signals can be activated correspondinglysuch that image data may be obtained during the relatively motionlessdiastole period.

A major drawback with prospective gating of MR signal acquisition asdescribed above is that such known methods fail to safely determine thetime of the diastole if either the heart cycle of the patient changesduring image acquisition or the patient simply has an irregular heartbeat. For example, the patient's holding of his or her breath duringimage acquisition may cause a significant change in the heart cycle. Ithas also to be taken into account in this context that patientssubjected to cardiac MR imaging often suffer from cardiovasculardisease, and cardiac arrythmia is one frequent symptom of cardiovasculardisease. Known cardiac MRI methods fail to produce images without motionartifacts in such cases.

Therefore it is readily appreciated that there is a need for an improvedcardiac MRI method which enables the acquisition and reconstruction ofhigh quality MR images of the heart in cases in which the heart cycle ofthe examined patient changes over time or is irregular because ofarrythmia. It is consequently the primary object of the presentinvention to provide a cardiac MR imaging method which makes sure thatin such cases MR signals used for image reconstruction are acquiredexclusively during the diastole period.

In accordance with the present invention, a method for cardiac magneticresonance imaging of the type specified above is disclosed, wherein theECG of the patient is monitored continuously during acquisition of MRsignals in step B), and wherein MR signals used for image reconstructionin step C) are retrospectively selected depending on the time intervalbetween successive R-waves.

The invention enables the reliable generation of high quality cardiac MRimages even if the duration of the heart cycle of the examined patienteither changes over time or is irregular because of cardiac arrythmia.This is achieved by the continuous monitoring of the ECG during imageacquisition. In correspondence with the detection of R-waves in the ECGsignal, only those MR signals are selected for image reconstructionwhich retrospectively appear to have been acquired during the quietdiastole phase of the heart cycle. It is made sure in this way that noMR signals are processed during image reconstruction by which blurringor other motion artifacts due to the beating of the heart would byintroduced into the final image. It is important to note in this contextthat the invention completely avoids the error-prone prospectiveestimation of the duration of the heart cycle as it is known in the art.

In accordance with the method of the invention it is-useful to acquirethe MR signals over a plurality of cardiac cycles or at least over morethan one cardiac cycle. With the known imaging sequences of RF pulsesand magnetic field gradient pulses it is often not possible to acquire acomplete set of MR signals as required for the reconstruction of thedesired MR image within a single cardiac cycle. Hence, the proposedmethod can easily be applied such that MR signals with different phaseencoding values used for reconstruction of a single image are acquiredduring the diastoles of two or more successive heart cycles.

It is known that the heart is in the relatively motionless diastolicphase after a certain delay time after the occurrence of an R-wave. Thevalue of the delay time can be determined according to a general rulewhich establishes a dependency on the average heart rate. Hence, thedelay time is usually adapted according to the average heart rate inconventional prospective gating methods. With the method of theinvention, the acquisition of MR signals for each cardiac cycleadvantageously begins after a delay period in an early diastolic phaseof the cardiac cycle after detection of an R-wave and may continue untilthe next R-wave is detected. In this way, the effective duration of theacquisition period per cardiac cycle is extended significantly incomparison to conventional prospective gating methods. This is achievedby using a shorter delay period and by continuing the signal acquisitionuntil the detection of the next R-wave. Hence, a maximum of imaging datacan be obtained depending on the duration of the individual heart cycle.

The proposed method offers different opportunities to select the optimalsignal acquisition interval during the time interval between successiveR-waves:

The optimal delay period for an average heart rate may deviate frompatient to patient Consequently, MR signals acquired during thediastolic phase can be retrospectively selected in accordance with themethod of the invention from a series of MR signal sets acquired withdifferent delay periods.

Alternatively, a further improvement in cases of patients suffering fromarrythmia can be achieved in accordance with the method of the inventionby generating a histogram of the detected heart rates during a scan andby adapting the delay period if the duration of the interval betweensuccessive R-waves deviates from the average heart rate. In this way,the value of the delay period is adjusted in accordance with theactually detected heart rate.

In accordance with the method of the invention, a phase encoding schememay be applied in step b) such that the phase encoding values of theacquired MR signals are repeated cyclically. The cyclic repetition ofphase encoding values facilitates the retrospective selection of anoptimum signal acquisition interval. This is because the repetition ofphase encoding values makes sure that a complete set of MR signals isavailable for image reconstruction irrespective of the actual selectionof MR signals which is made retrospectively as described above.

The imaging technique of the invention works best if the amplitudes ofthe individual acquired MR signals are approximately constant. Hence,the method of the invention may advantageously be applied in combinationwith a so-called steady state acquisition scheme as it is known in theart as such. Preferably, the sequence of RF pulses and magnetic fieldgradient pulses used for MR signal acquisition may be a balanced turbofield echo (TFE) sequence.

It is easily possible to incorporate the method of the present inventionin a dedicated device for magnetic resonance imaging of a body of apatient placed in a stationary and substantially homogeneous mainmagnetic field. Such an MRI scanner comprises means for establishing themain magnetic field, means for generating magnetic field gradientssuperimposed upon the main magnetic field, means for radiating RF pulsestowards the body, control means for controlling the generation of themagnetic field gradients and the RF pulses, means for receiving andsampling magnetic resonance signals generated by sequences of RF pulsesand magnetic field gradient pulses, reconstruction means for forming animage from said signal samples, and means for monitoring the ECG of thepatient. In accordance with the invention, the control means, which isusually a microcomputer with a memory and a program control, comprises aprogramming with a description of an imaging procedure according to theabove-described method of the invention.

A computer program adapted for carrying out the imaging procedure of theinvention can advantageously be implemented on any common computerhardware, which is presently in clinical use for the control of MRIscanners. The computer program can be provided on suitable datacarriers, such as CD-ROM or diskette. Alternatively, it can also bedownloaded by a user from an internet server.

The following drawings disclose preferred embodiments of the presentinvention. It should be understood, however, that the drawings aredesigned for the purpose of illustration only and not as a definition ofthe limits of the invention.

In the drawings

FIG. 1 shows a diagram of the imaging procedure-in accordance with theinvention;

FIG. 2 shows an embodiment of an MRI scanner of the invention.

FIG. 1 illustrates the cardiac MR imaging method of the invention. Thefigure shows an ECG signal of a patient with three R-waves designated byR₁, R₂, and R₃. The individual heart cycles are determined by the timeintervals between successive R-waves R₁, R₂, and R₃. As can be seen inFig. 1, the heart cycle of the examined patient changes over time. Thisirregularity might for example be due to the patient suffering fromcardiovascular disease and cardiac arrythmia. In accordance with theinvention, the depicted ECG signal is monitored continuously during theacquisition of MR signals, and the R-waves R₁, R₂, and R₃ are detectedautomatically, for example by means of a computer and an appropriateprogram which evaluates the digitized ECG signal. The MR signalsacquired according to the method of the invention are designated by theletters a,₁₋₃, b₁₋₃, c₁₋₃, d₁₋₃, and e₁₋₃. The letters a, b, c, d, and erepresent the cyclically repeated phase encoding values of the MRsignals. The sequence of RF pulses and magnetic field gradient pulsesused for generation of the MR signals is not shown in the figure. Inaccordance with the method of the invention, the acquisition of MRsignals for each cardiac cycle begins after a delay period D afterdetection of each R-wave R₁, R₂, R₃. The signal acquisition thencontinues until the respective next R-wave is detected. MR signals usedfor image reconstruction are retrospectively selected after 8 signalacquisition depending on the time interval between successive R-wavesR₁, R₂, and R₃.

In the depicted case, diagonally hatched MR signals d₁ to c₁, b₂ to a₂,and c₃ to b₃ are selected in accordance with the invention.

In Fig. 2 a magnetic resonance imaging device 1 is diagrammaticallyshown.

The apparatus 1 comprises a set of main magnetic coils 2 for generatinga stationary and homogeneous main magnetic field and three sets ofgradient coils 3, 4 and 5 for superimposing additional magnetic fieldswith controllable strength and having a gradient in a selecteddirection. Conventionally, the direction of the main magnetic field islabelled the z-direction, the two directions perpendicular thereto thex-and y-directions. The gradient coils are energized via a power supply11. The apparatus 1 further comprises a radiation emitter 6, an antennaor coil, for emitting radio frequency (RF) pulses to a body 7, theradiation emitter 6 being coupled to a modulator 8 for generating andmodulating the RF pulses. Also provided is a receiver for receiving theMR signals, the receiver can be identical to the emitter 6 or beseparate. If the emitter and receiver are physically the same antenna orcoil as shown in Fig. 2, a send-receive switch 9 is arranged to separatethe received signals from the pulses to be emitted. The received MRsignals are input to a demodulator 10. The modulator 8, the emitter 6and the power supply 11 for the gradient coils 3, 4 and 5 are controlledby a control system 12 to generate the above-described sequence of RFpulses and a corresponding sequence of magnetic field gradient pulses.The control system is usually a microcomputer with a memory and aprogram control. For the practical implementation of the invention itcomprises a programming with a description of an imaging procedureaccording to the above-described method. The demodulator 10 is coupledto a data processing unit 14, for example a computer, for transformationof the received echo signals into an image that can be made visible, forexample, on a visual display unit 15. There is an ECG means 16 formonitoring the ECG of the patient 7 during acquisition of MR signals,which may be for example a standard digital ECG recording device,connected to the control system 12. The ECG means 16 in turn isconnected to the patient 7 via a cable and appropriate electrodes.

1. Method for cardiac magnetic resonance imaging, the method comprisingthe following steps: A) monitoring the ECG of a patient and detectingthe occurrence of R-waves in the ECG; B) acquiring a plurality of phaseencoded MR signals per cardiac cycle by subjecting the patient to asequence of RF pulses and magnetic field gradient pulses; C)reconstructing an MR image from at least a part of the MR signalsacquired in step B); wherein the ECG of the patient is monitoredcontinuously during acquisition of MR signals in step B), wherein MRsignals used for image reconstruction in step C) are retrospectivelyselected depending on the time interval between successive R-waves. 2.Method for cardiac magnetic resonance imaging according to claim 1,wherein the MR signals are acquired over a plurality of cardiac cycles.3. Method for cardiac magnetic resonance imaging according to claim 2,wherein the acquisition of MR signals for each cardiac cycle beginsafter a delay period (D) in an early diastolic phase of the cardiaccycle after detection of an R-wave and/or continues until the nextR-wave is detected.
 4. Method for cardiac magnetic resonance imagingaccording to claim 3, wherein the delay period (D) is determined fromthe average time interval between successive R-waves.
 5. Method forcardiac magnetic resonance imaging according to claim 3, wherein MRsignals acquired during the diastolic phase are retrospectively selectedfrom a series of MR signal sets acquired with different delay periods.6. Method for cardiac magnetic resonance imaging according to claim 3,wherein the delay period is adapted in accordance with the actuallydetected duration of the interval between successive R-waves.
 7. Methodfor cardiac magnetic resonance imaging according to claim 1, wherein aphase encoding scheme is applied in step B) such that the phase encodingvalues of the acquired MR signals are repeated cyclically.
 8. Method forcardiac magnetic resonance imaging according to claim 1, wherein thesequence of RF pulses and magnetic field gradient pulses is selectedsuch that a steady state acquisition scheme is applied.
 9. Method forcardiac magnetic resonance imaging according to claim 8, wherein thesequence of RF pulses and magnetic field gradient pulses is a balancedturbo field echo sequence.
 10. Device for magnetic resonance imaging ofa body of a patient placed in a stationary and substantially homogeneousmain magnetic field, the device comprising means for establishing themain magnetic field, means for generating magnetic field gradientssuperimposed upon the main magnetic field, means for radiating RF pulsestowards the body, control means for controlling the generation of themagnetic field gradients and the RF pulses, means for receiving andsampling MR signals generated by sequences of RF pulses and magneticfield gradient pulses, reconstruction means for forming an image fromthe signal samples, and means for monitoring the ECG of the patient,wherein the control means comprises a programming with a description ofan imaging procedure according to a method of claim
 1. 11. Computerprogram with a program code, wherein the program code enables an imagingprocedure according to a method of claim 1 to be carried out on amagnetic resonance device.